This invention relates generally to magnetic resonance imaging (MRI).
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials, and provides medical imaging. It is generally non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
MRI is based on nuclear spins, which can be viewed as vectors in a three-dimensional space. During an MRI process, each nuclear spin responds to four different effects: precession about the main magnetic field, nutation about an axis perpendicular to the main field, and both transverse and longitudinal relaxation. In steady-state MRI processes, a combination of these effects occurs periodically.
Compared with other modalities, such as X-ray, CT and ultrasound, MRI takes longer time, sometimes several minutes, for data acquisition to generate clinically useful images. Undesirable imaging artifacts may appear due to the long scan time. MRI using multiple receiving coils (phased array) has been introduced to shorten the scan time and increase signal to noise ratio (SNR). This fast imaging technique, known as parallel imaging (PI), can significantly accelerate data acquisition, and therefore reduce imaging artifacts and improve image quality.
Motion is a major source of artifacts for Magnetic Resonance (MR) studies. A typical sequence prescribed on the scanner takes anywhere from a couple seconds to a number of minutes. As a result, the scan is sensitive to motion. Motion can come from any number of sources including respiration, cardiac motion, blood flow, and even unintentional patient movement. The effects have been long studied and have been typically observed as ghosting, intensity changes, and blurring.